Ban

Banning a client means that you are not going to upload or download files to/from him. That means that a client which has been banned will just be ignored.
A client can be banned because of leeching or because it supposed a potential damage to the network or the client, such as Agressive clients.

It is important to notice that the basic difference between a ban and a leech is that, while a ban doesn't neither upload nor download, a leech doesn't upload, but does download.